As a result of the recent rapid increase in network traffic, WDM (Wavelength Division Multiplexing) technology capable of transmitting large volumes of information has come to be widely used. Also, to meet requirements such as reduction in cost, there has been a demand for all-optical transmission over long distances without the conversion to electricity in the middle.
Meanwhile, in an optical fiber used for optical transmission, the velocity of light varies depending on the wavelength, so that a phenomenon called chromatic dispersion occurs in which optical wavelengths emitted simultaneously from the transmitting end arrive at the receiving end at different times. Chromatic dispersion leads to distortion of the received waveforms, and if the distortion caused is too large, the conveyed information fails to be accurately discriminated.
Thus, optical signals propagated through an optical fiber are passed through a DCM (Dispersion Compensation Module) having chromatic dispersion characteristics opposite in sign, to compensate the chromatic dispersion. This makes it possible to transmit optical signals over a long distance while restraining the waveform distortion.
However, the amount of chromatic dispersion increases in proportion to the transmission distance, and also different types of optical fibers have different dispersion characteristics. Accordingly, dispersion compensation modules with different dispersion compensation characteristics are needed for different transmission distances and for different types of optical fibers.
Generally, the dispersion compensation module is constituted by a passive component such as an optical fiber, and thus it is very often the case that a single variety of dispersion compensation module has a fixed dispersion compensation characteristic. It is therefore necessary to determine in advance the locations on an optical network where individual dispersion compensation modules with certain dispersion compensation characteristics are to be arranged.
Determining the locations of individual dispersion compensation modules with certain dispersion compensation characteristics on an optical network in this manner is referred to as layout design (dispersion compensation design) for the dispersion compensation modules on the optical network.
In WDM communications, on the other hand, multiplexed transmission is performed using a plurality of different wavelengths. The necessary amount of dispersion compensation varies depending on the wavelength channel because of the chromatic dispersion, and therefore, when carrying out the dispersion compensation design, it is necessary that the chromatic dispersion of an optical transmission line be compensated over the wavelength range used, that is, the range from the longest wavelength channel through to the shortest wavelength channel.
It is, however, difficult to compensate the chromatic dispersion of an optical transmission line over a wide wavelength range by means of a single dispersion compensation module. Namely, the extent to which the chromatic dispersion is compensated varies from one wavelength channel to another, with the result that some wavelength channels are undercompensated while others are overcompensated.
If residual chromatic dispersion that failed to be compensated falls within an certain range, a transmission device can correctly receive information. In practice, therefore, the dispersion compensation is designed such that the residual chromatic dispersion of all wavelength channels that failed to be compensated falls within an allowable chromatic dispersion range. As conventional dispersion compensation design techniques, a technique has been proposed in which the amount of dispersion compensation is set so that the residual dispersion may fall within an allowable residual dispersion range (cf. Domestic re-publication of PCT international application No. 2005/006604 (page 6, line 46 to page 8, line 26, FIG. 6)).
With respect to a path which is routed from the start to the end point of optical transmission over a WDM optical network, the dispersion compensation is ordinarily designed such that the residual dispersion (remaining dispersion that failed to be compensated by dispersion compensation modules) of a specified path falls within an allowable range.
FIG. 18 illustrates a path on an optical network. The path p1 starts from a node n1 and terminates at a node n4 and is routed from the node n1 to the node n4 via nodes n2 and n3. In the case of a system in which a dispersion compensation module is arranged at the receiving end, dispersion compensation modules are arranged in the respective nodes n2, n3 and n4 (in this instance, a dispersion compensation module is arranged in each of the nodes n2, n3 and n4, though, in practice, it is unnecessary to provide each receiving end with a dispersion compensation module).
The dispersion compensation module arranged in the node n2 compensates the chromatic dispersion caused on an optical fiber f1. The dispersion compensation module arranged in the node n3 compensates the chromatic dispersion caused on an optical fiber f2, and the dispersion compensation module arranged in the node n4 compensates the chromatic dispersion caused on an optical fiber f3.
To obtain the amount of chromatic dispersion (residual dispersion) at the end point of a certain path, the chromatic dispersions of the individual optical fibers from the start to the end point of the path are added up, then the chromatic dispersions of the individual dispersion compensation modules on the path are added up, and the sums obtained are added together, thereby obtaining the residual dispersion at the end point of the path.
In the case of the path p1, for example, the chromatic dispersions of the optical fibers f1 to f3 from the start point to the end point are added up, then the chromatic dispersions of the dispersion compensation modules arranged in the nodes n2, n3 and n4 are added up, and the obtained sums are added together.
Namely, the sum total of the chromatic dispersion amount of the optical fiber f1, the chromatic dispersion amount of the dispersion compensation module arranged in the node n2, the chromatic dispersion amount of the optical fiber f2, the chromatic dispersion amount of the dispersion compensation module arranged in the node n3, the chromatic dispersion amount of the optical fiber f3 and the chromatic dispersion amount of the dispersion compensation module arranged in the node n4 corresponds to the amount of chromatic dispersion at the end point (node n4) of the path p1, namely, the residual dispersion of the path p1.
When designing the dispersion compensation, the residual dispersion is calculated in the aforementioned manner with respect to each of specified paths input to a dispersion compensation design tool, and how dispersion compensation modules with respective dispersion compensation characteristics are to be laid out within an optical network is determined such that the residual dispersion falls within the allowable range.
Making the residual dispersion fall within the allowable range means causing the residual dispersion to fall within the range defined by desired upper and lower bounds (upper- and lower-bound dispersion values) at the end point. If the upper bound is exceeded by the residual dispersion, then the wavelength is undercompensated, and if the residual dispersion is smaller than the lower bound, the wavelength is overcompensated.
Also, when calculating the residual dispersion in the manner stated above, conventionally the calculation is performed only with respect to a single reference wavelength, among multiple wavelengths multiplexed on a WDM signal. For example, where there are 40 different wavelengths λ1 to λ40 to be multiplexed, the residual dispersion is calculated using a center wavelength λ20 or a common wavelength 1550 nm as the reference wavelength.
Conventionally, moreover, a fixed-value dispersion variation range (hereinafter referred to as fixed variation range) is set which is estimated from the calculated residual dispersion of the reference wavelength to include the residual dispersions of other wavelengths than the reference wavelength, and the dispersion compensation is designed such that the fixed variation range falls within an allowable range.
FIG. 19 illustrates a dispersion map (transition of cumulative dispersion with distance) of the path p1 depicted in FIG. 18, wherein the horizontal axis indicates distance and the vertical axis indicates dispersion amount.
It is assumed that a WDM signal on which 40 wavelength channels λ1 to λ40 are multiplexed is transmitted along the path p1, and that the reference wavelength with respect to which the residual dispersion is calculated is λk. It is also assumed that the residual dispersion of the reference wavelength λk at the node n4 is calculated to be rk, and that the residual dispersions of the shortest and longest wavelengths λ1 and λ40 at the node n4 are calculated to be r1 and r40, respectively.
A fixed variation range B is estimated which contains the variation range of the residual dispersion rk plus margins, and the dispersion compensation is designed such that the fixed variation range B falls within the allowable range (between the upper and lower bounds).
If the fixed variation range B is within the allowable range, the path is judged to be capable of transmission, and if the fixed variation range B is outside the allowable range, the path is judged to be incapable of transmission. Also, the compensation values and locations of dispersion compensation modules are determined so that the number of paths capable of transmission may be maximized.
In the case illustrated in FIG. 19, the residual dispersion rk of the reference wavelength λk as well as the residual dispersions r1 and r40 of the shortest and longest wavelengths λ1 and λ40 are included in the fixed variation range B, and also the fixed variation range B falls within the allowable range. Consequently, the path p1 is judged to be capable of transmitting a 40-wavelength WDM signal.
The fixed variation range B is, however, no more than an estimated value based on the single reference wavelength. Since, in practice, the variation ranges of the individual wavelengths vary depending on the dispersion compensation amounts and characteristics of the dispersion compensation modules, there is no guarantee that the variation ranges of all wavelengths will certainly fall within the estimated fixed variation range B in a real optical network.
FIG. 20 illustrates a dispersion map, wherein the horizontal axis indicates distance and the vertical axis indicates dispersion amount. A variation range b at the node n4 indicates an actual variation range of the reference wavelength, and as illustrated, the variation range b is shifted toward the upper bound and falls outside the fixed variation range B. In this case, the residual dispersion r1 of the wavelength λ1 possibly exceeds the upper bound.
Thus, with the conventional method, it is not possible to determine with accuracy whether the dispersions of the individual wavelength channels certainly fall within the allowable range, and in some cases, the actual variation ranges of certain wavelengths fall outside the estimated fixed variation range B, in which case an erroneous judgment may possibly be made that the path is capable of transmission, though it is actually not.
To optimally design the dispersion compensation, it is desirable that the residual dispersion variation ranges of the individual wavelengths be minimized. According to the conventional dispersion compensation design described above, however, variation of the chromatic dispersion is obtained on the basis of the single reference wavelength, and dispersion variations of the other wavelengths are estimated to fall within the fixed variation range which is so set as to include the dispersion variation of the reference wavelength plus margins. Thus, with the conventional techniques, it is not possible to carry out optimum design in such a manner that actual chromatic dispersion variations of the individual wavelengths are minimized.